HaloSat Observations of Heliospheric Solar Wind Charge Exchange
Abstract
X-ray emission from solar wind charge exchange (SWCX) produced in interplanetary space contaminates every astrophysical observation, regardless of the line of sight. Unfortunately, the primary SWCX emission lines also happen to be important diagnostics of astrophysical plasmas. Models of SWCX emission are limited by two main uncertainties: the local solar wind fluxes along the line of sight and the charge exchange cross sections. The He cone, a localized density enhancement of helium neutrals, is the only heliospheric SWCX emission feature that is small enough and bright enough to be observationally isolated from the X-ray background and the broader SWCX emission. HaloSat, an X-ray CubeSat mission, has recently completed two series of specialized observations, near and far from the ecliptic plane, during two Earth transits of the He cone. These observations were used to test the predictions of an SWCX emission model against the emission observed at low ecliptic latitude, where the solar wind data are monitored, and at high ecliptic latitude, where the solar wind data are extrapolated. The measured SWCX emission for the set of observations near the ecliptic plane was consistent with the line intensities predicted by the model but underpredicted for the set of observations at high ecliptic latitude near the south ecliptic pole. Additionally, high-temperature Galactic halo emission components are reported for both spectral sets.